1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and apparatus for using electricity to set a gap in a piezoelectric pressure sensor during fabrication of the piezoelectric pressure sensor.
2. Discussion of the Background
A hydrophone may be a device for listening to and recording sound underwater. For example, a hydrophone may be used for reflective seismology, recording sound bounced off the floor of a body of water in order to determine the structure and composition of the earth under the floor. A hydrophone may use a piezoelectric pressure sensor as a transducer for detecting sound and generating a recordable signal. In U.S. Pat. No. 7,382,689, the entire disclosure of which is incorporated herein by reference, a flexible hydrophone using one or more active elements, or piezoelectric pressure sensors, for detecting seismic signals underwater, is disclosed. FIG. 1 depicts an exemplary cross-section of a piezoelectric pressure sensor. The piezoelectric pressure sensor 101 may be constructed in part by attaching a piezoelectric flex element 102, such as, for example, a piezoceramic unimorph, to the top of a tray 105. FIG. 2 depicts an exemplary piezoelectric flex element for a piezoelectric pressure sensor. The piezoelectric flex element 102 may include a piezoelectric material 103, such as, for example, a piezoceramic, bonded to an inactive substrate 104, which may be metal, glass, plastic or any other suitable material FIG. 3 depicts an exemplary tray for a piezoelectric pressure sensor. The tray 105 may be any suitable shape, including, for example, rectangular, and be made from metal, or any other suitable material. The tray 105 may include a floor 108, and a step 107. The step 107 may encircle the inside of the tray 105, and provide a surface for attaching the piezoelectric flex element 102 to the tray 105.
The piezoelectric flex element 102 may deflect when subjected to pressure, resulting in the piezoelectric material 103 accumulating an electrical charge that can be used to drive current in circuit. The degree to which the piezoelectric flex element 102 can deflect in response to hydrostatic pressure may need to be limited in order to, for example, protect against damage to the piezoelectric flex element 102. This may be accomplished by providing a mechanical limit, such as a rigid surface which may stop the piezoelectric flex element 102 from deflecting any further once the piezoelectric flex element 102 has deflected far enough to contact the rigid surface. The floor 108 of the tray 105 on which the piezoelectric flex element 102 is mounted may serve as the rigid surface. During fabrication of the piezoelectric sensor 101 a gap 106 between the floor 108 and the bottom of the piezoelectric flex element 102 may be established. The size of the gap 106 between the piezoelectric flex element 102 and the floor 108 may be the desired distance the piezoelectric flex element 102 may be allowed to deflect before being mechanically limited. The gap 106 may be large enough to allow the piezoelectric flex element 102 to deflect unrestricted in response to operating pressures within the normal range for the piezoelectric pressure sensor 101.
Setting the gap 106 during the fabrication of the piezoelectric pressure sensor 101 may be difficult, as the gap 106 may be only a few millimeters in size or smaller. If the gap 106 is not maintained properly the piezoelectric flex element 102 may end up too low in the tray 105, leaving no room for the piezoelectric flex element 102 to deflect in response to pressure, or the piezoelectric flex element 102 may break. This may limit the ability of the piezoelectric pressure sensor 101 to detect pressure.
Current fabrication processes may maintain the gap 106 mechanically during fabrication of the piezoelectric pressure sensor 101. FIG. 4 depicts an exemplary cross section of a piezoelectric pressure sensor with a mechanically maintained gap in accordance with the prior art. As depicted in FIG. 4, not at scale, a shim 401 may be placed on the floor 108 of the tray 105 before the piezoelectric flex element 102 is placed on the tray 105. The shim 401 may be made of any suitable material, such as, for example, plastic. The shim 401 may maintain the gap 106 between the bottom of the piezoelectric flex element 102 and the floor 108 by physically preventing the bottom of the piezoelectric flex element 102 from going any lower than the top of the gap 106 while adhesive between the piezoelectric flex element 102 and the tray 105 is cured. Once the adhesive has cured and the piezoelectric flex element 102 is adhered to the tray 105, locking in the gap 106, the shim 401 may be removed. Inserting and removing the shim 401 may require extra steps during the fabrication process. An access point, such as a hole in the tray 105, may be needed to remove the shim 401, and the access point may need to be sealed up once the shim 401 has been removed. These extra steps may add to the cost and complexity of fabricating the piezoelectric pressure sensor 101.
Thus, there is a need to have a method and apparatus for more efficiently setting a gap during the fabrication of piezoelectric pressure sensors.